Atomic Absorption and Atomic Fluorescence

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Atomic Absorption andAtomic Fluorescence

• Yongsik Lee

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Introduction

• AAS and AFS similarity
• Sample introduction
• Atomization
• AFS has not gained widespread general use for
  routine elemental analysis

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9A. Sample atomization tech

• Flame atomization
• Electrothermal atomization
• Specialized atomization techniques
• Glow discharge atomization
• Hydride atomization
• Cold vapor atomization

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Flame Atomization

• Neubulization
• the sample solution is dispersed into tiny
  droplets.
• Desolvation
• the solvent of the solution is evaporated.
• The finely divided solid aerosol is mixed with a
  fuel and an oxidant.
• common fuels natural gas, hydrogen, acetylene
• common oxidants air, oxygen, nitrous oxide
• Volatilization
• The sample is burned in a flame produced by the
  fuel and oxidants to form gaseous molecules. 
• Temperatures ranging from 1700 C to 3150 C are
  produced depending on the fuel/oxidant
  combination.
• At such a high temperature, the gaseous molecule
  (MX) can be
• atomized (MX --gt M)
• and ionized (M --gt M). 
• Energy of a particular wavelength will be used to
  excite the molecule, atom, and ions.  Changes in
  the energy level can be measured for quantitative
  determination.

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Scheme of flame atomization
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Types of Flames

• Fuel
• Natural gas
• Hydrogen
• Acetylene
• Oxidant
• Air 1700-2400 ?
• Oxygen 2500-3100?
• Nitrous oxide 2500-3100 ?
• Table 9-1 Properties of Flames
• Burning velocity important for flame stability
• Flashback flame propagate back into the burner
• Blowing off the burner at higher flow rates

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Flame structure

• Flame profile
• Figure 9-2 9-3
• Primary combustion zone
• Blue luminescence of C2, CH, and other radicals
• Internal region
• Rich in free atoms
• Secondary combustion zone
• Products of the inner core are converted to
  stable molecular oxides

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Temperature profile

• Figure 9-3
• Max temperature
• Location about 1 cm above the primary combustion
  zone
• Optical focus to this region

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Flame absorbance profiles

• Ag (not readily oxidized)
• Continuous increase from the flame
• Cr (forms stable oxides)
• Continuous decrease
• Oxide formation dominant
• Mg
• Have a maximum
• Use proper portion of the flame for maximum
  absorbance

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Flame atomizerlaminar flow burner
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Multiple-slot premixed CH4/air burner
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Flame Diagnostics

• Laser-induced fluoresence measurements of OH, CH,
  CN and H2CO radicals in low and atmospheric
  pressure flames
• rag.web.psi.ch/htdz/Www_Homepage/
  Comb_Diag_Exp.ht

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Performance Characteristics

• Reproducible behavior
• Best to all other methods for liquid samples
• Sampling efficiency ( Sensitivity)
• bad
• Sample flows down the drain
• Residence time in the optical path in the flame
  is brief ( 1/10000 s)

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Electro-thermal Atomization

• An Electro-thermal Atomizer consists a
  cylindrical graphite tube connected to an
  electrical power supply.  
• A small amount (0.5 to 10mL) of sample is
  introduced and heated electrically.
• Two stage heating
• At lower temperature the sample is evaporated
  and ashed
• Rapid increase in temperature volatilization and
  atomization
• Used in ICP, AAS, AFS

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Electrothermal atomization

• Advantage
• less reproducible results than flame atomization
• Atomizie in short time
• residence time in the optical path is long(lt1sec)
• Higher sensitivity

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Electrothermal Atomizers

• entire sample atomized short time (2000-3000 C)
• sample spends up to 1 s in analysis volume
• superior sensitivity (10-10-10-13 g analyte)
• less reproducible (5-10 )
• Flame method - 1 or better
• Used when flame or plasma atomization provides
  inadequate detection limit

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ETA
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Electrothermal Graphite Furnace

• Gas flow
• external Ar gas prevents tube destruction
• internal Ar gas circulates gaseous analyte
• Three step sample preparation for graphite
  furnace
• Dry - evaporation of solvents (10-gt100 s)
• Ash - removal of volatile hydroxides, sulfates,
  carbonates (10-100 s)
• Fire/Atomize - atomization of remaining analyte
  (1 s)

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ETA output signal

• sample
• Canned orange juice 2 mL
• Drying 20 s
• Ashing 60 s
• Standards lead
• High speed data acquisition possible
• Rapid (lt1 s) response
• Quantitative analyses
• Based on peak height
• Peak area is also used

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9B Atomic Absorption Instrumentation

• AAS should be very selective
• each element has different set of energy levels
• lines very narrow
• BUT for linear calibration curve (Beers' Law)
  need bandwidth of absorbing species to be broader
  than that of light source difficult with ordinary
  monochromator
• Solved by using very narrow line radiation
  sources
• minimize Doppler broadening
• pressure broadening
• lower P and T than atomizer
• and using resonant absorption
• Na emission 3p2s at 589.6 nm used to probe Na in
  analyte

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Radiation Sources

• Hollow cathode lamp
• The most common source for AAS
• W anode, cylindrical cathode of specific metal,
  1-5 torr Ne or Ar
• Electrodeless discharge lamp

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Hollow cathode lamps

• 300 V applied between anode and metal cathode (-)
• Ar ions bombard cathode and sputter cathode atoms
• Fraction of sputtered atoms excited, then
  fluoresce
• Cathode made of metal of interest
• Na, Ca, K, Fe...
• different lamp for each element
• restricts multi-element detection
• Metal mixture
• Hollow cathode
• to maximize probability of redeposition on
  cathode
• restricts light direction
• High potentials (high currents)
• Lead to greater intensities
• Self-absorption by unexcited atoms

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Electrodeless Discharge Lamps

• Greater Radiation intensity
• 10-100 times than HCL
• Sealed quartz tube a few torr of Ar metal (or
  its salt)
• Light by RF (27 MHz) or microwave radiation
• Ionization of Ar to excite the metal
• Less reliable than HCL

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Jedis holding EDLs
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Spectrophotometers

• Single beam design
• Dark current is nulled with a shutter
• 100 T adjustment with a blank is aspirated into
  the flame

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Double beam design AAS
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Scheme of double beam

• Beam usually chopped or modulated at known
  frequency
• Signal then contains constant (background) and
  dynamic (timevarying) signals

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Interferences in AAS

• Signal at one wavelength often contains
  luminescence from interferents in flame
• Spectral interferences
• Chemical interferences

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Interferences

• Spectral interferences
• Spectrum is close (AA lines in 0.1 Å)
• Cannot be resolved by the monochromator
• Chemical interferences
• By chemical processes during atomization
• Alter the absorption characteristics

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Chemical interferences

• reverses atomization equilibria
• reacts with analyte to form low volatility
  compound
• releasing agent - cations that react
  preferentially with interferent - Sr acts as
  releasing agent for Ca with phosphate
• protecting agent - form stable but volatile
  compounds with analyte (metal-EDTA formation
  constants)

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Ionization equilibria

• Ionization of atoms and molecules
• Can be neglected in air flame
• Significant in higher temperatures
• hotter atomization means
• more ionization
• emission from interferents

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Degree of ionization at flame
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Detection limit

• AA/AE comparable (ppb in flame)
• AAS less suitable for
• weak absorbers (forbidden transitions)
• metalloids and non-metals (absorb in UV)
• metals with low IP (alkali metals)

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Detection limit 1-20 ng/mL
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Interferences in AAS

• Signal at one wavelength often contains
  luminescence from interferents in flame
• Spectral interferences
• Chemical interferences

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Spectral interference

• Overlap of atomic spectral lines
• Very rare for atoms
• V line at 3082.11Å with Al line at 3082.15 Å
• Use Al line at 3092.7 Å
• Combustion by-products other than analyte and
  particulate
• most significant
• fuel and oxidant interferences
• Background correction can correct those
• Sample matrix interferences more challenging
• Oxides and hydroxides may need to change flame
  chemistry
• Organic material species incomplete combustion
  products scattering
• Most dependent on flame chemistry and temperature
• Add excess to standards for radiation buffer
  effect

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Correction of spectral interference

• Historically, spectral interference in graphite
  furnace most severe, very specialized corrections
  applied to minimize problem
• Two Line Correction
• An additional spectral line from the source,
• close in frequency to the analyte wavelength can
  be employed.
• Impurity in HLC, NE or Ar in HLC, or sample
  (non-resonant emission)
• special case rare
• Continuum Source Correction
• Signal from a continuous (deuterium lamp) is
  alternately passed through the analyte zone.
• Limited value.
• Zeeman Effect Correction
• In a strong magnetic field (10 KG), the magnetic
  field generated by the spinning electron alters
  the energy or wavelength of transitions.
• For Singlet transitions, 3 lines -s, p, s
  result. p lines absorb radiation polarized
  parallel to magnetic field, s perpendicular to
  field
• Smith-Hieftje correction

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Zeeman Effect Magnetic
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The magnet of our love
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Zeeman effect correction

• B - Rotating polarizer
• E absorption at // polarization
• Very sensitive correction technique

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Atomic Absorption TechniquesUS EPA method 7000A

• 4.8 Glassware
• All glassware, polypropylene, or Teflon
  containers, including sample bottles, flasks and
  pipets, should be washed in the following
  sequence
• detergent, tap water, 11 nitric acid, tap water,
  11 hydrochloric acid, tap water, and reagent
  water.

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US EPA method 7000A

• 5.7 Calibration standards
• preparation of standards which produce an
  absorbance of 0.0 to 0.7.
• Calibration standards are prepared by diluting
  the stock metal solutions at the time of
  analysis.
• calibration standards should be prepared fresh
  each time a batch of samples is analyzed.
• Prepare a blank and at least three calibration
  standards in graduated amounts in the appropriate
  range of the linear part of the curve. The
  calibration standards should be prepared using
  the same type of acid or combination of acids and
  at the same concentration as will result in the
  samples following processing.
• Calibration curves are always required.

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US EPA method 7000A

• 7.2 Direct aspiration (flame) procedure
• In general, after choosing the proper lamp for
  the analysis, allow the lamp to warm up for a
  minimum of 15 minutes
• Align the instrument, position the monochromator
  at the correct wavelength,
• select the proper monochromator slit width, and
  adjust the current according to the
  manufacturer's recommendation. Subsequently,
• Light the flame and regulate the flow of fuel and
  oxidant. Adjust the burner and nebulizer flow
  rate for maximum percent absorption and
  stability.
• Run a series of standards of the element under
  analysis.
• Construct a calibration curve by plotting the
  concentrations of the standards against
  absorbances.
• Aspirate the samples and determine the
  concentrations either directly or from the
  calibration curve.
• Standards must be run each time a sample or
  series of samples is run.

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US EPA method 7000A

• 8. QUALITY CONTROL
• 8.2 A calibration curve must be prepared
• each day with a minimum of a calibration blank
  and three standards.
• After calibration, the calibration curve must be
  verified by use of at least a calibration blank
  and a calibration check standard (made from a
  reference material or other independent standard
  material) at or near the mid-range.
• The calibration reference standard must be
  measured within 10 of it's true value for the
  curve to be valid.

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US EPA method 7000A

• 8. QUALITY CONTROL
• 8.3 If more than 10 samples per day are analyzed,
• the working standard curve must be verified by
  measuring satisfactorily a mid-range standard or
  reference standard after every 10 samples.
• This sample value must be within 20 of the true
  value, or the previous ten samples reanalyzed.

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• 8.6.1 Dilution test
• For each analytical batch select one typical
  sample for serial dilution to determine whether
  interferences are present.
• The concentration of the analyte should be at
  least 25 times the estimated detection limit.
• Determine the apparent concentration in the
  undiluted sample.
• Dilute the sample by a minimum of five fold and
  reanalyze.
• Test results
• Agreement within 10 between the concentration
  for the undiluted sample and five times the
  concentration for the diluted sample indicates
  the absence of interferences, and such samples
  may be analyzed without using the method of
  standard additions.

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EPA Test Methods

• Q What are test methods? A Test methods are
  approved procedures to measure
• the presence and concentration of physical and
  chemical pollutants
• evaluating properties, such as toxic properties,
  of chemical substances
• or measuring the effects of substances under
  various conditions.
• Q Why an index?A This Index was developed to
  improve access to US EPA test methods. It is not
  an official EPA publication nor does inclusion or
  exclusion of methods indicate EPA approval or
  disapproval of any method. http//www.epa.gov/ne/o
  arm/index.html .

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What information is included?

• Method Number
• the official method number or a compiler assigned
  number if un-numbered.
• Examples 0330.1 or 3810 or TO-15 or 8080A (
  indicates not available) or SAMPLIN.
• Chemical or Method Description
• chemical, analyte, group of chemicals or name of
  protocol.
• If you don't find the one you want, try a broader
  term such as metals for mercury or pesticides for
  DDT.Examples asbestos or absorption or Maneb or
  larval survival or mercury.
• Reference Source of where to get the method by
  one of four categories
• EPA Report - EPA report number. Examples
  600/4-82-029 or SW-846 Ch 3.3.
• 40 CFR Part - Title 40 of the Code of Federal
  Regulations part numbers.Examples 136 App A
  40 CFR 136 Appendix A.
• Region 1 - EPA Region 1 Library local call
  number.Examples 01A0006125a or 01A0006706.
• Electronic Version - abbreviated reference to an
  electronic version if available.The full web
  address is provided in the Sources of EPA Test
  Methods list.Examples www or ttn/emc or NEMI
  the full web address provided in the Source List
  CD indicates included in the Water Methods CD ROM
  (EPA 821/C-99-004).
• Date Issued - date method was published.Examples
  09/25/1996 or /// (date unknown).

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Homework

• 9-12, 9-20, 9-21